Turbulent and transitional flows around airfoils near stalling conditions may be characterized by low-frequency oscillations during which the flow alternates between attached and detached states, or by stall cells, that induce a spanwise modulation of the flow on the suction side of the airfoil. In this paper we investigate the onset of such phenomena for the transitional flow over a NACA0012 airfoil at the Reynolds number Re=90000 using global stability analysis. The transitional nature of the flow is modelled using the linear eddy-viscosity model developed by Spalart & Allmaras that is coupled here with a correlation-based algebraic transition model. Steady-state solutions of the aforementioned system reveal a characteristic inverted-S shape curve with two saddle-node bifurcations. The global stability analysis of these steady solutions is based on the full linearization of the governing discrete equations, including the turbulence model, the transition model and the numerical stabilization terms. It reveals the existence of two unstable modes: a two-dimensional low-frequency (unsteady) mode and a three-dimensional zero-frequency (steady) mode. The competition between these two modes is investigated by following their corresponding eigenvalues along the branches of steady solutions. We have identified the critical angles for each mode and shown that the three-dimensional modes become unstable prior to the two-dimensional ones, for this particular case.